The principal aim of this research is to examine the role of ductal and alveolar hyperplastic lesions in the genesis of chemical and viral-induced mammary tumors. Over the past 5 years, several different model systems have been developed which include the evolution of tumors from chemical carcinogen-induced ductal preneoplastic dysplasias and from viral- and/or hormonal-induced alveolar preneoplastic lesions. The normal, preneoplastic, and neoplastic cells have been investigated in homotypic cell culture conditions to examine morphological, cytological, and cell surface-related properties indicative of the neoplastic state. These experiments suggest that most of the characteristics unique to neoplastic cells in vitro are associated with neoplastic progression rather than neoplastic transformation. Experiments are currently in progress to examine the effects of specific inhibitors/promotors of carcinogenesis on the growth and developmental potential of preneoplasias utilizing combined in vivo/in vitro experiments.